Aulacoseira humilis

Frustules are cylindrical and form short colonies. Valves are 5-9 µm in diameter, with a mantle height between 2-5 µm. The ratio of the mantle height to valve diameter is less than 1. The mantle has straight sides and the valve face is often strongly convex or concave. The valve face has large, round areolae in a marginal ring and the area inside this ring has similar but irregularly positioned areolae. These valve face areolae have a density of 17 per 10 µm. The straight pervalvar rows of areolae are positioned in pairs, within grooves. There are 24-40 rows of areolae in 10 µm. The smaller areolae are often not visible in LM, but a pair of areolae can appear together as a single, large areola, which would give the striae an apparent density of 12-20 per 10 µm. The density of the areolae along each row is between 16-22 per 10 µm. Terminating each pair of rows is a single, large areola. The spines are located at the end of each pervalvar costa. The spines are long and thin, tapering away from the valve face, have an anchor-shaped end and may be up to 3 µm long. Linking and separation spines are not differentiated. The ringleiste is solid, shallow and broad.

Florin (1981) notes that in Cleve-Euler’s (1951) examination of this taxon she found that the diameter can be as great as 12 µm. The striae in Florin (1981), at 10 per 10 µm, are coarser than what we observed. Siver and Kling (1997) note that the tips of the spines are slightly swollen rather than anchor-shaped. They also found that most of the valves had convex valve faces and a few had flat faces, but make no mention of concave faces.